1. Field of the Invention
The present invention relates to small, compact heat sinks that include microchannels with coolant flowing therethrough. More specifically, the present invention relates to heat sinks with a wide thermal range that includes cryogenic temperatures so that the coolers are suitable for cooling to superconductor temperatures. The invention also relates to cooling integrated circuits.
2. Description of Related Art
Heat generation is a common problem with semiconductor devices such as integrated circuits. Temperature buildup can reduce the lifetime of semiconductor components, change their electrical characteristics, and at high temperatures, sufficiently degrade the semiconductor junction to render the circuit useless. Most consumer electronic devices rely on passive cooling, or use fans to cool electrical components. However, these cooling means are inadequate for high performance circuits, such as those that must dissipate a very large amount of power, or for closely packed circuits, or circuits that are designed to function extremely quickly. In such circuits, heat buildup is a factor that can limit system performance. If available, a more aggressive, more powerful cooling means can be used to provide better performance. Active cooling means, including forced coolant flow systems, have been used with integrated circuits. For example, a so-called "thermal conduction module", comprising a complicated structure including pistons and springs, is presently used in IBM products.
Microchannels, which are small microscopic channels formed in silicon wafers, have been disclosed to be effective heat sinks for integrated circuits. When a coolant is forced through such microchannel coolers, it has been demonstrated that a large amount of heat can be removed from a small area. For example, Tuckerman, in U.S. Pat. No. 4,573,067, discloses a semiconductor chip including microscopic channels defined by fins in intimate contact with the chip. The microscopic channels are enclosed by a cover, to enclose the channels. Fluid flow through the channels is disclosed to be approximately laminar.
Microchannels themselves have received much attention. However, little attention has been focused on the means for delivery of coolant to the microchannels. A prior art microchannel cooling system is illustrated in the exploded view of FIG. 1. A top silicon layer 10, a middle glass layer 12, and a plastic manifold layer 14 are connected together to form a cooler assembly 16. The underside of the top layer 10 comprises silicon microchannels 20. The middle layer 12 comprises two glass slots 22, 24 that extend therethrough. The bottom plastic layer 14, which is connected to the coolant delivery tubes 26, 28, includes a cavity 30,32. In operation, coolant flows from the tube 26, through the cavity 30 in the plastic layer 14, and into the slot 22. From there, the coolant then flows through the microchannels 20, absorbing heat. The heated coolant flows into the other slot 24, through the cavity 32 in the bottom layer 14, and out the tube 28.
In the prior art configuration of FIG. 1, several problems have arisen that limit the cooling performance of the system. One problem is the leaks that frequently appear at the connection between the plastic layer 14 and the middle layer 12. This problem raises reliability issues at low pressures and restricts the upper pressure limit of the prior art heat sink. Higher pressure means more cooling; thus the leaks represent a limit on the effective cooling available. An effective bond between plastic and glass is not readily achievable, and thus this connection has a propensity to leak. In contrast, the silicon-glass bond between the top layer 10 and the middle layer 12 is very strong, and can withstand much more pressure than the glass-plastic connection. Thus, the glass layer 12 was commonly used as a cover for the microchannels 20.
Another problem with the prior art is an insufficient thermal range-at very hot or cold temperatures the prior art heat sink of FIG. 1 is easily susceptible to breakage and leaking. Thermal range is limited by the mismatch in thermal expansion coefficients between the plastic layer 14 and the glass layer 12. This mismatch problem increases in importance as the temperature deviates significantly from room temperature. The effect of an expansion mismatch is a degradation of the structural integrity, the possibility of cracking, and an increase in the likelihood of leaks. Another weak part in the prior art heat sink is the tube insertion point-the connection between the heat sink and the coolant tube. Conventionally, this connection is accomplished by cementing the heat sink to the coolant tube. Cracking and leaking of coolant from these connections is common, even within a narrow temperature range.
Generally, the thermal range of the prior art is very limited. The problem is particularly acute at high temperatures, for example 250.degree. C., or at cryogenic temperatures. Liquid nitrogen can be maintained at -196.degree. C., and it would be an advantage to provide a microchannel heat sink that can effectively cool with liquid nitrogen as the active coolant. Such a cooler would have uses in superconducting technology, which requires cryogenic temperatures to maintain their superconductivity, Such a cooler may also have use in supercomputers, which have a high density of components and short connections, thereby requiring a great amount of cooling power. Presently, supercomputer circuits are immersed in a cooling fluid, an arrangement which provides some cooling; but not a great deal due to the limited thermal conductivity between the fluid and circuits. Another problem with direct immersion is the reliability of the circuits is compromised by fluid leaks and the corrosiveness of the fluid. It would be an advantage to provide effective cooling of supercomputer circuits with a coolant that is isolated from the electrical components, and that has much higher thermal conductivity than immersion cooling.